Rollup merge of #34060 - JDemler:master, r=steveklabnik
Rollup merge of #34060 - JDemler:master, r=steveklabnik

Improved documentation for tests/ directory

This ambigouity problem was already discussed in the [forums](https://users.rust-lang.org/t/problem-using-external-modules-inside-integration-test-submodule/5312/6).

graydon/rust,graydon/rust,aidancully/rust,GBGamer/rust,aidancully/rust,aidancully/rust,GBGamer/rust,GBGamer/rust,aidancully/rust,GBGamer/rust,GBGamer/rust,graydon/rust,GBGamer/rust,graydon/rust,aidancully/rust,GBGamer/rust,graydon/rust,GBGamer/rust,graydon/rust,aidancully/rust